D-block contraction

d-block contraction (sometimes called scandide contraction) is a term used in chemistry to describe the effect of having full d orbitals on the group 13 elements to highlight the effect on gallium.

Gallium can be seen to be anomalous. The most obvious effect is that the sum of the first three noble gas electron core. In contrast to the group 13 elements this sequence shows a smooth reduction.

Other effects of the d-block contraction are that the Ga³⁺ ion is smaller than expected, being closer in size to Al³⁺. Care must be taken in interpreting the ionisation potentials for indium and thallium, other effects e.g. the inert pair effect become increasingly important for the heavier members of the group.
The cause of the d-block contraction is the poor shielding of the nuclear charge by the electrons in the d orbitals. The outer valence electrons are more strongly attracted by the nucleus causing the observed increase in ionization potentials. The d-block contraction can be compared to the lanthanide contraction which is caused by inadequate shielding of nuclear charge by electrons occupying f orbitals.

References

Greenwood, N. N.; Earnshaw, A. (1997). Chemistry of the Elements, 2nd Edition, Oxford:Butterworth-Heinemann. ISBN 0-7506-3365-4.